An X-ray computed tomography apparatus (hereinafter referred to as CT apparatus) is known to provide images of information from an examination object, such as a patient, (hereinafter referred to as object) based on the intensity of an X-ray transmitted through the object. The images provided by the CT apparatus play an important part in a lot of medical practices including a diagnosis of a disease, a treatment, and a medical operation planning. Imaging by the CT apparatus is now realized by, for example, a multi-slice scanning and a helical scanning as the latest technique. The multi-slice scanning is a technique of obtaining a plurality of projection data of the object in a single rotation scanning. The helical scanning is a technique of obtaining projection data of a wide range of the object by scanning the object in a helical manner.
A multi-slice scanning CT apparatus usually requires alignment of its multi-slice X-ray detector, as needed, in the following exemplary manner, so as to keep a preferable scanning condition.
FIG. 1 is an example showing a physical relationship between an X-ray tube and an X-ray detector in a conventional multi-slice CT apparatus according to a prior art. In FIG. 1, a slit 200 may be adjusted, for example, so that an X-ray irradiated from an X-ray tube 204 can be exposed to an X-ray detector 202 in order to obtain four slices each of which has a 2 mm width (i.e. 2 mm×4 slices) in a direction of the object body axis in each scanning. The X-ray detector 202 may obtain an output value α in the above condition. Further, the slit 200 may also be adjusted, for example, so as to obtain four slices each of which has an 8 mm width (i.e. 8 mm×4 slices) in each scanning, and under this condition an X-ray irradiated from the X-ray tube 204 can be exposed to the X-ray detector 202 which is prepared to detect four slices each of which has a 2 mm width (i.e. 2 mm×4 slices), in each scanning. The X-ray detector 202 may obtain an output value β in the above condition. It is a conventional technique of alignment between the X-ray tube 204 and the X-ray detector 202 that a ratio α/β is approximated to 1 in every detecting segment of the X-ray detector 202. In this alignment, it may be possible to adjust and/or confirm whether the X-ray irradiated from the X-ray tube 204 is exposed to a predetermined operative area of the X-ray detector 202 or not (whether the predetermined operative area of the X-ray detector 202 in the direction of the object's body axis is covered by an umbra of the X-ray irradiated from the X-ray tube 204).
According to the above-mentioned conventional alignment, however, it is not possible to adjust to match a central axis of the X-ray irradiated from the X-ray tube 204 and the center of the X-ray detector 202 in the direction of the object's body axis. Therefore, an aperture width of the slit 200 may be required to be wider than a necessary width of the aperture in each data acquisition mode (for example, 2.0 mm×4 slices in a single rotation scanning). This may cause the object to be exposed to radiation excessively.